Design and Implementation of a fixed-frequency Adapter...

Design and Implementation of a fixed-frequency Adapter with Very

Low Power Consumption

Agenda• New ENERGY STAR® requirements• Needed features to meet the new specification• New controller family NCP1237/38/87/88• Design step 1: Power stage• Design step 2: Set the compensations• Design step 3: No Load Input Power• Design step 4: Magnetics• Design step 5: EMI• Preliminary demo board example• Conclusion

EPA 2.0 (External Power Supplies)

(was > 0.84 in previous version 1.1)

(< 0.5 W in 1.1)

(< 0.75 W in 1.1)

EPS 5.0 (ENERGY STAR® Program Requirements for Computers)

• Defines ETEC for different types of products as a Typical Energy Consumption• For the desktop and notebook product categories TEC will be determined by the following formula:

ETEC = (8760/1000) * (Poff * Toff + Psleep * Tsleep + Pidle * Tidle)

• where all Px are power values in watts, all Tx are Time values in % of year, and the TEC ETEC is in units of kWh and represents annual energy consumption based on mode weightings

• The light load efficiency and no load consumption is more important

Category A: ≤ 40.0 Category B: ≤ 53.0 Category C: ≤ 88.5

Category A: ≤ 148.0 Category B: ≤ 175.0Category C: ≤ 209.0Category D: ≤ 234.0

TEC (kWh)

Notebook Computers (kWh)Desktops and Integrated Computers (kWh)

ETEC requirement desktops and notebooks

• Effective from July 1, 2009 (except: game consoles from July 1, 2010)

• Ways to improve efficiency:

– Lower the switching frequency FSW frequency foldback at light loads

– Lower the Drain voltage at turn-off valley switching

Improving Efficiency• Sources of loss:

– Switching losses:

– Losses caused by leakage inductance:

SWoffturnDRAINDRAINswitchingloss FVCP ⋅⋅⋅= −2

)()( 21

SWpeakleakloss(leak) FILP ⋅⋅⋅= 2

Reducing No-load Input Power• Static losses in the start-up circuit:

– Start-up resistor permanently drawing current from the bulk capacitor

• Ways to lower the start-up circuit losses– With external start-up resistor Extremely low start-up current– Integrated start-up current source Extremely low leakage when off– Connect the start-up circuit to the half-wave rectified ac input

HV rail

NCP1351 Start-up resistors

10.0M 30.0M 50.0M 70.0M 90.0Mtime in secs

CbulkVcc

RstartupHW

CVccauxiliarywinding

Mainsinput

10.0M 30.0M 50.0M 70.0M 90.0Mtime in secs

CbulkVcc

RstartupHW

CVccauxiliarywinding

Mainsinput

4startup

startupHW

π= Brings a 21% reduction in power

startupstartupHW

Selected frombulk connection

Reducing No-load Input Power

NCP1237/38/87/88The NCP1237/38/87/88 series represents the next generation of fixed frequency PWM controllers. It targets applications where cost-effectiveness, reliability, design flexibility and low standby power are compulsory.

High-voltage current source with built-in Brown-out and mains OVP Freq. reduction in light load conditions and skip modeAdjustable Over Power Protection

Fewer components and rugged designExtremely low no-load standby powerSimple option to alter the max. peak current set point at high line

AC-DC adapters for notebooks, LCD monitor, game console, printersCE applications (DVD, STB)

NCP1237/38xDR2G - NCP1287/88xDR2GSOIC-7 2500p per reel

Others Features

Ordering & Package InformationMarket & Applications

Latch-off input for severe fault conditions, allowing direct connection of NTCTimer-based protection: auto-recovery or latchedDual OCP option availableBuilt-in ramp compensationFrequency jittering for a softened EMI signatureVcc operation up to 30 V

Pb O, DW

Unique Features Benefits

Value Proposition

Application Data

Various options available depending upon end applications needs

DSS Dual OCP

Latch Auto Recovery

NCP1237A Yes Yes YesNCP1237B Yes Yes YesNCP1238A Yes No YesNCP1238B Yes No YesNCP1287A HV only Yes YesNCP1287B HV only Yes YesNCP1288A HV only No YesNCP1288B HV only No Yes

Avail. in Q1 2010

NCP1237/38/87/88 – Built-in Startup FET

No startup resistor!

Low initial startup current to prevent damage if Vcc pin is shorted to ground.

High startup current to reduce charging time

A flyback auxiliary winding supplies biasing voltage in normal condition to save power

Saves PCB area& saves power

How it works...How it works...

NCP1237/38/87/88 – Dynamic Self Supply (Optional)

3ON/OFF

10.5V/12V

10.00M 30.00M 50.00M 70.00M 90.00M

11.3V avgVccon

Vccoff

Currentsource

Power ON Current Source turns ON Vcc is rising; no output pulsesVcc reaches Vcc(on) Current Source turns OFF Vcc is falling; output is pulsingVcc falls to Vcc(off) Current Source turns ON Vcc is rising; output is pulsing

No need of auxiliary winding!Dynamic Self-Supply

NCP1237/38/87/88 – Dual Startup Current Level

Startup current is low initially to prevent damage when VCC pin is grounded.

Startup current is activated when VCCdrops to VCC(off). Hence, the voltage never drops below VCC(off) after startup.

Startup current is off when VCCreaches VCC(on)

NCP1237/38/87/88 – Brown-out and Mains OVP

Can be connected to thehalf-wave rectified ac line

Detection independent ofRipple on HV pin

NCP1237/38/87/88 – Brown-out and Mains OVP

Passes full line cycle drop-outTimer-based detection

One Shot

VHV(start)

Starts only at VCC(on)

VHV(stop)

HV timerstarts

HV timer

restarts

40ms min.

Brown-out

The compensation currentcreates an offset on theCurrent Sense signal

NCP1237/38/87/88 – Over Power Protection

Over Power Protection Maximum output power clamped

Need to compensate for theeffect of the propagation delay

NCP1237/38/87/88 – Dual OCP Threshold

Accommodates large output power transients Suitable for printers

Fault timer starts

Output load

Max output power

Fault timer starts

Output load

Max peak

Over load

Over loadTransient

peak power

Output still in regulation

Max DC

power0.7 V at CS pin

0.7 V at CS pin

0.5 V at CS pin

These protections use the Up/Down counters, like classical analog integration.

NCP1237/38/87/88 – 4 ms Soft Start

with soft-start

voltage / current without soft-start

VCCMax. current setpoint envelope

4 ms “digital” soft-start operation

Stressless start-up phase4 ms Soft Start

NCP1237/38/87/88 – Frequency Foldback

Increased efficiencySwitching frequencylowered at light load

No audible noiseSwitching frequencyclamped at 25 kHz

NCP1237/38/87/88 – Recover from Standby

Improved Load Transient response time

Transient Load Detect Function (TLD)

Soft-Skip mode is left as

soon as the voltage on the feedback pin reaches the

TLD threshold

NCP1237/38/87/88 – Latch-off Protection

VLATCH

Latch!

Less external components needed

An NTC thermistor can be directly connected to the IC

• There is a built in slope compensation with no external setting• The internal slope compensation is activated if the duty cycle is higher

than 40%• The amount of slope compensation is 5mV/% observed at CS pin

Duty Cycle

Internal PWM setpoint

40% 100%80%

VILIMITVILIMT – 0.2 V

NCP1237/38/87/88 – Slope Compensation

Application Schematic

Power stage: Schematic of Flyback Converter

primNN

N sec=

Power Stage Design: Bulk Capacitor

outoutout IVP ⋅=

inavgin V

• Output power Pout

disavginbulk V

⋅= ,

• Estimation of input power Pin

• Average input current Iin,avg

• Bulk capacitor value Cbulk

Estimate the η based on the EPA standard

Use tdis=8.5ms

Power Stage Design: Drain Voltage

toff ton

tleakV

Vds(t)

Vds_max

Power Stage Design: Transformer Ratio

( )max,max,

2085.0 bulkDS

diodefoutC

VVVVVk

N−−⋅

V diodefoutr

min,max

Transformer ratio – consideration of the VDSS of used Q1

Reflected voltage Vr at primary from secondary

Maximum duty cycle DCmax

clampC V

In CCM operation: In DCM operation doesn’t depend on N:

min,min,max

swprim

DC⋅⋅

The 20V means margin for clamping diode turning-on overshoot.

primNN

N sec=

Power Stage Design: Current Ripple

The average shared transformer current reflected to primary winding IL,avg

II avgin

avgL =

Choose the relative ripple δIr: it affects the operation in the CCM or DCM

avgLr I

avgLr III ,⋅=Δ δ

valleypeak III −=Δ

⎟⎠⎞

⎜⎝⎛ +⋅=

ravgLpeak

III δ

⎟⎠⎞

⎜⎝⎛ −⋅=

ravgLvalley

I valle

I peak

• For universal AC input design use the δIr in range 0.5 to 1.0• For European AC input use the δIr in range 0.8 to 1.6

Power Stage Design: Primary Inductance

bulkprim Δ⋅

⋅= maxmin,

⎟⎟⎠

⎞⎜⎜⎝

⎛ Δ+Δ⋅−⋅=

maxIIIIDCI peakpeakprimRMS

I peakpeak =sec, N

II Δ=Δ sec

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛ Δ+Δ⋅−⋅−=

secsec,2

sec,maxsecI

IIIDCI peakpeakRMS

Transformer primary winding inductance Lprim

Maximum RMS value of the current flowing through primary winding Iprim,RMS

Maximum RMS value of the current flowing through secondary winding Isec,RMS

Power Stage Design: Q1 Selection

Then the right device is chosen by parameters VDSmax, Ipeak, ton, toff

Conduction loss at Q1 should be approx. 1% of the Pout

2,100 RMSprim

outDSon I

⋅≤

Current sensing resistor Rsense selection

ILIMsense I

1.1 senseprimRMSsense RIP ⋅= 2

The 1.1 factor means 10% margin for Lprim and other parameters spread, to be able to deliver maximum power.

Power Stage Design: Secondary Rectification

outbulk VNVPIV +⋅= max,

The next important parameters for D1 selection are Isec,peak, Iout and the fast and soft recovery

D1 selection: Cout selection:

rippleout

ESRsec,

22sec,, outrmsrmsCout III −=

Reflected voltage across D1 Minimum Cout value

The maximum allowed ESR of Cout

it is recommended to use more parallel Cout for lowering the output voltage ripple.

Dominant part

swrippleout

outout FV

⋅⋅

Power Stage Design: Clamping NetworkTVS – losses in the suppressor:

RCD clamp – 1st iteration:

rclamp

clampswpeakleakswclampclamp VV

VFILFEP

−⋅⋅⋅⋅=⋅= 2

swpeakleak

clampleakclamp FIL

⋅⋅

⋅⋅= 2

clampclamp R

swclampripple

clampclamp FRV

⋅⋅>

These values need to be optimized for the no load consumption and losses in slow clamping diode D2

better EMI response

better at no load conditions

TVS vs RCD Clamp Comparison

Drain voltage ringing with TVS as clamp Drain voltage ringing with RCD as clamp

Ch1 – Drain, Ch3 – Clamp node

Different Rdamp used in clamp

Synchronous Rectification

• New SR controller NCP4303 coming in 2010

Over Power Compensation

OPPOPPoff

OPPOPP

PROPbulk

CSPEAK R

RgVRRg

RVI ⋅⋅+⎟⎟

⎞⎜⎜⎝

⎛⋅−⋅+= int

sensePROPOPP gL

⋅⋅

The overpower compensation affects the primary peak current, by the following formula:

Then the overpower compensation resistor can be calculated:

The over power compensating resistor affects only the Ipeak value, but in CCM the output power is given by the following formula, where Ivalley plays a role:

valleypeakswprimout IIFLP −⋅⋅⋅⋅= η

2nd Level Over Power Protection

( )sense

OPPOPPoffbulk

CStranTRAN R

RgVVRVI ⋅⋅−−=

The overpower compensation affects the 2nd level over power protection by the addition of bulk voltage feed forward.

The overpower compensation can be used for reducing the transformer size to ½ and keeping the peak power capability.

Spread Sheet Design of OPC

Inputs: Output voltage Vout [V] 19Primary turns N1 [-] 100Secondary turns N2 [-] 25Ramp Comp at CS RaCo [mV/%] 5Maximum int set point Vilimit [V] 0.7Sensing resistor Rsense [Ohm] 0.235Propagation delay tprop [ns] 100Primary inductance Lp [uH] 560Vin to Iopp ratio gopp [uS] 0.5Over power comp resistor Ropp [Ohm] 680Switching frequency Fsw [kHz] 652nd level overcurrent prot Vcstran [V] 0.5

OPC design spread sheet was created and the user can choose the right ROPP and it’s effect to Ipeak, Itran, Pout and Ptran:

Will be available soon, while NCP1237/38/87/88 will be released

Spread Sheet Design of OPCPmax & Ptran vs line input voltage

0 50 100 150 200 250 300 350 400

Vbulk [V]

Ptran Pmax

Ipeak & Itran vs line input voltage

0 50 100 150 200 250 300 350 400

Vin [V]

], Itr

Ipeak Itran

Keeping constant Ipeak in CCM mode tends to Ivalley decreasing with increasing the Vin. That’s why the maximum output deliverable power Pout increases with increasing Vin. Choose the right compensation.

Loop Compensation

Download the work sheet at:

http://www.onsemi.com/pub/Collateral/FLYBACK DWS.XLS.ZIP

No Load Input Power Reducing Approach

• Decrease the transformer leakage inductance• Use the controller IC with the frequency foldback and skip mode features• Do not allow the DSS operation (Vcc cap increase)• In case of low Vcc and high aux winding leakage increase the aux number of turns

to disable the DSS• Decrease the value of the Vcc damping resistor (may affect the EMI)

No Load Input Power Reducing Approach

• Lower the M1 switching losses• Optimize the clamping circuitry• Reduce the losses in the secondary rectifier and its snubber• Decrease the TL431 biasing• Decrease the cross current through the feedback resistor divider• Set a stable operation for all loading currents• Do not use the output voltage indication LED

Area Product AP

• There is defined the area product AP [m4]• Product of effective window area Wa [m2] and iron cross

section area Ac [m2]

caP AWA ⋅=

• Allows fast, effective and optimal magnetic design• Should be published in core datasheet

Window Utilization Factor KuKu is a measure of the amount of copper that appears in the window area of transformer. This window utilization factor is affected by:

1) Wire insulation2) Wire lay (fill factor)3) Bobbin area4) Insulation required for multilayer windings or between windings

Typical values lay in range 0.35 to 0.48

The Load Coefficient Kload

• Flux density in magnetic should be designed at Ipeak with some margin (5%) to avoid saturation

• Do you really need 100% Iout for 100% time??

If not, decrease core size!!

RMSout

RMSoutload I

Example: • Maximum DC output current is 3.5 A, but it’s only needed for

transients• The long term RMS value is only 1.75 A (at least 10 min.)• Loading coefficient is only 0.5 (not 1) → core size is smaller → losses in core and in copper are smaller

Flyback Transformer Core SizingThe core size can be calculated by the AP factor in case of these inputs:

1. Converter parameters: Lprim, Ipeak, Kload, δIr, DCmax2. Core maximum flux density Bmax considered with the hysteresis and

eddy current losses at switching frequency Fsw3. Winding parameters (utilization factors for primary and secondary

windings Kuprim, Kusec), (current densities in primary and secondary windings Jprim, Jsec

Now the appropriate core can be selected from the vendor products list by the AP factor .

secsec

maxmax

23121+⋅+

⋅⎟⎟⎠

⎞⎜⎜⎝

⋅−

⋅⋅⋅

primprimload

peakprimP I

Windings Design

• Number of turns of primary winding

peakprimprim AB

primNTNNT ⋅=sec

, NTVVVV

NTdiodefout

VccfCCaux ⋅

• Number of turns of secondary winding

• Number of turns of auxiliary winding

Air Gap Length lg

−⋅⋅

MPL – core magnetic path length

μ0 - permeability of vacuum

μr - permeability of core

MPLlg <<in case of

In case an EE, RM or pot core is used, divide the calculated lg by factor 2, because your core has 2 air gaps in magnetic path

How to improve EMI of my design?

AC line filter

Diode snubber

DC output filter

DRV dampingPower switch loop

Clamping loop

• All switching loops with RF currents should have small area

• Divide input AC filter at two chokes to decrease the parasitic capacitance coupling

• CY – closes the current loop for the RF currents injected via transformer

Diode Snubber Design• Snubber resistance value should be close to the characteristic

impedance of ringing circuitry

SECleaksnubber C

• RC time constant of the snubber should be small compared to the switching period but long compared to the voltage rise time

dsnubber CC ⋅÷≈ 43

Lleak,SEC –the transformer leakage inductance observed from secondary side

Cd – reverse direction diode capacitance

PCB Layout Tips

Clamping loopDRV loop

Output loop Capacitor pads arrangement for better filtering RF currents

Power switch loop

Diode snubber

Agenda• New ENERGY STAR® requirements• Needed features to meet the new specification• New controller family NCP1237/38/87/88• Design step 1: Power stage• Design step 2: Set the compensations• Design step 3: No Load Input Power• Design step 4: Magnetics• Design step 5: EMI• Preliminary demonstration board example• Conclusion

Preliminary Demonstration Board

(optimized for EPS 2.0)

A typical 65 W notebook adapter (19 V output)

Schematic of Preliminary Demonstration Board

A typical 65 W notebook adapter (19 V output)

(optimized for EPS 2.0)

Demonstration Board Efficiency (Measured with DC Cord)

85.96 %

87.88 %

87.63 %

87.37 %87.10 %100 %(65 W)

87.52 %75 %(49 W)

87.79 %25 %(16 W)

87.54 %50 %(32 W)

230 Vac/60Hz115 Vac/60HzVIN

% of POUTnom

Average at 115Vac is 87.32% and at 230 Vac is 87.21 %

The DC cord length is 1.05m and copper cross sec. is 0.75mm2

Demonstration Board Standby PowerLight load and no load input power with the NCP1237

73.5 mW

73.77 %

78.72 %

83.74 %86.55 %10 %(6.5 W)

85.40 %5 %(3.3 W)

51.1 mWNo load

77.49 %1 %(0.65 W)

230 Vac/50 Hz115 Vac/60 HzVIN

0 10 20 30 40 50 60 70

Pout [W]

]Demonstration Board Efficiency

– 115 Vac - NCP1237– 230 Vac - NCP1237

Demonstration Board Conducted EMI

80% of full load (2.72 A) at 230 V/50 Hz NCP1237B 65 kHz

NCP1237B 100 kHz Frequency Jittering

Ref1 – DRV frequency

NCP1237B 100 kHz Frequency Foldback

Ch1 – DRV, Ch2 – FB, Ref1 – DRV frequency

Load Transient Response from 20% to 100%

Ch1 – Drain, Ch2 – FB, Ch3 – Vout (AC coupling), Ch4 - Iout

Load Transient Response from 100% to 20%

Ch1 – Drain, Ch2 – FB, Ch3 – Vout (AC coupling), Ch4 - Iout

Conclusion• Meeting the most recent requirements from ENERGY STAR®

or IEC is possible with the classical Flyback converter

• The new controller NCP1237/37/87/88 with frequency foldback and skip-mode at light load makes it possible

• Average efficiencies above 87% are possible

• No-load input power below 300 mW is possible• No-load input power below 100 mW is achievable, although

the controller alone cannot ensure this. The whole power supply must be designed to reduce power waste.

For More Information

• View the extensive portfolio of power management products from ON Semiconductor at www.onsemi.com

• View reference designs, design notes, and other material supporting the design of highly efficient power supplies at www.onsemi.com/powersupplies

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